Words have meaning. And while in the real world you can say "Yes, sir" and modulate your tone from anywhere between full subverience to open rebellion, on blogs and forums the only communication possible is verbal. Thus if you use words without adhering to a commonly understood meaning, you are basically just talking to yourself, or maybe a small fan club of people you taught "your" meaning of the words you are using.
A moron is a person with an IQ of 51 to 70. Which makes him cleverer than an imbecile (IQ 26 to 50), and much cleverer than an idiot (IQ 0 to 25). But these terms aren't used scientifically any more, because they became commonly used as insults. But words having meaning is true for insults as well. If you call somebody a "jerk", you not only indicate that you wish to insult that person, you are also saying that it was some sort of not nice behavior that made you choose this particular insult. Choosing "moron" indicates that you think that the other person is not very intelligent.
The reason why "moron" is not a good word to use in World of Warcraft to describe bad players is that there is little or no correlation between intelligence and how well a person plays WoW. Joke videos apart, it is almost certain that Stephen Hawking would be extremely bad at playing World of Warcraft, in spite of an IQ of 160. But even children with mental handicaps can become surprisingly good a playing MMORPGs. Neither World of Warcraft, nor any other MMORPG, are actually intellectually challenging. Training and dedication are much stronger correlated with performance in MMORPGs than intelligence is. The highest performance can in some cases only be reached if you *don't* think about what you are doing, but rely on muscle memory instead. The pinnacle of MMORPG performance is in raiding, and apart from the very first raiders who actually have to develop a strategy, the process of raiding does not require anything more than average intelligence, and good reaction time.
By specifically insulting a bad player's intelligence, you also express your belief that the situation is permanent, that this bad player will never become a good player, because of hardwired intelligence limitations. But in reality whatever keeps the bad player from doing better is almost certainly not hardwired. It might be a simple lack of training, a lack of time, or a lack of motivation. But if you take the guy who is currently doing 1k dps in a pickup group and offer him a million dollars if in 6 months he can get his damage output up to 10k, you would most certainly lose your money. Given time and motivation, anybody who got as far as just joining a heroic is also able to do very well.
The big disadvantage of reacting to any sort of bad behavior with insults is that this approach actively prevents you from coming up with constructive criticism. Even if we bloggers are just armchair game designers, identifying the reasons for why something doesn't work as we want wrongly can only lead to us proposing a wrong solution to the problem, or none at all. If you claim that the "morons" are holding World of Warcraft back, the only solution you could come up with is requiring an IQ test before being allowed to play the game. And as that is not the real reason for people playing or behaving badly, that solution would not improve anything.
Once you look at the "morons" discerningly, and really identify *why* they are behaving as they do, you can come up with a game design for a MMORPG which would actually be better. Game designers can fix bad behavior with social engineering and incentives, so you could lobby them to do so, if you had an actual solution. They can't fix the intelligence of their players. And honestly, they aren't even likely to listen to somebody who just throws around insults. As the philosophists say, "To insult is to assert or assume dominance, either intentionally claiming superiority or unintentionally revealing lack of regard." That only attracts a similarily trash-talking audience.